Fly ash is one of the residues generated as coal is combusted. Fly ash is the particulate matter that rises with the flue gases leaving the combustion chamber. In the past, fly ash was typically released into the atmosphere. Now, however, fly ash has been recognized as an environmental pollutant and is generally captured using electrostatic precipitators or other filtration equipment. In the United States, fly ash is generally stored at coal power plants or placed in a landfill. Two classes of fly ash are categorized according to ASTM C618: Class C fly ash and Class F fly ash. The primary difference between the classes is the amount of calcium, silica, alumina, and iron content in the fly ash, which are largely determined by the chemical content of the coal burned. The burning of hard, old anthracite and bituminous coal typically produces Class F fly ash. Class C fly ash is produced from the burning of young lignite or subbituminous coal.
Fly ash has been used as a component in cement and supplemental cement materials. Most of the research to date has been conducted using Class F fly ash. In order to form cement materials using Class F fly ash, large amounts of an alkali “activator” (e.g., NaOH) are typically required. An aqueous solution of an alkali activator (e.g., >4.5 M NaOH) is mixed with the fly ash. The use of large amounts of alkali activators raises concerns, however. First, alkali activators are electrochemically generated by the chloralkali process, which produces with harmful Cl2 or HCl gas by-products. Second, the presence of free alkali can have deleterious effects on activated fly ash cement. Third, the production of large amounts of alkali is an energy intensive process and can indirectly lead to increased toxic CO2 emissions.